1. Field of the Invention
This invention relates to a method for manufacturing a nitride semiconductor laser element and a nitride semiconductor laser element.
2. Background Information
There is growing demand for semiconductor laser elements in which a compound semiconductor, for example, nitride semiconductors expressed by the general formula InxAlyGa1-x-yN (0≦x, 0≦y, 0≦x+y≦1) are used, in a wide variety of applications, such as in optical disk systems capable of recording and reproducing large volumes of information at high density (such as a next-generation DVD), or in electronic devices such as personal computers. Therefore, considerable research has been devoted to using nitride semiconductors to manufacture semiconductor laser elements with good reproducibility while maintaining stable characteristics.
For example, a method in which grooves that extend in a cavity direction of a laser element and is formed from the upper face of a nitride semiconductor growth layer to the interface constituting a pn junction has been proposed as a method for preventing chipping, pulverization, etc., of a ridge that would otherwise be caused by propagation from a region of concentrated dislocation in the substrate if a cavity end face is formed by cleavage, and thereby reducing damage to the nitride semiconductor laser element and protecting the current-voltage characteristics (see, JP-2004-327879-A).
However, even when the cleavage of a cavity end face is performed substantially perpendicular to these grooves, the effect of the dislocation density, crystal defects, etc., within the nitride semiconductor layer or substrate can cause the cleavage plane to be distorted from its intended position, making it difficult to obtain a sufficiently stable yield.
Usually, when a nitride semiconductor laser element is produced, a semiconductor layer and electrodes are formed in a wafer state, after which the wafer is divided into bars (hereinafter also referred to as “primary cleavage”), and the semiconductor layer bars are divided into chips (hereinafter also referred to as “secondary cleavage”). Consequently, if the primary cleavage is distorted from its intended position, a laser element of the desired cavity length will not be obtained, and this greatly affects the characteristics. Also, when the division is made where the electrodes are formed, the electrodes may droop down to the cavity end face, and this adversely and markedly affects the characteristics. Furthermore, it is difficult to divide the semiconductor layer bars into chips, and this greatly affects the yield.
Also, usually, cleaving into bars forms the cavity end face of the nitride semiconductor laser element, and an end face protective film is formed on the cavity end face. That is, light is emitted from the cavity end face formed in primary cleavage. Accordingly, high accuracy is required in primary cleavage, or, to put it another way, a smooth cavity end face needs to be formed.
Dislocations and crystal defects are generally present in a nitride semiconductor. When a laser element is produced from a nitride semiconductor, a problem is that leakage caused by these dislocations and crystal defects can lower the voltage in the minute electric current region and result in poor current and voltage characteristics. If the start-up voltage is low, this leads to problems such as a shorter element service life and poor electrostatic discharge.